//java
    public static int marsExploration(String s) {
    // Write your code here
    int changedLetters = 0;
    char[] chars = s.toCharArray();
    for(int i=0; i<s.length(); i+=3) {
        if(chars[i] != 'S')
            changedLetters++;
        if(chars[i+1] !='O')
            changedLetters++;
        if(chars[i+2] !='S')
            changedLetters++;
    }
    return changedLetters;
}